During the processing of a substrate in the plasma processing chamber, satisfactory results often require tight control of the process parameters. This is particularly true for processes such as deposition, etching, cleaning, etc., that are employed to manufacture modern high density integrated circuits.
In the execution of certain etch processes, for example, the plasma needs to be stable and well-characterized before the actual etch step may be performed on the substrate. To initiate a stable and well-characterized plasma, a special recipe known as a strike-step recipe is often employed. During this strike step, a relatively high gas pressure is employed in the plasma processing chamber to ensure plasma ignition. Radio frequency (RF) power is often kept low to prevent inadvertent damage to the substrate and/or the chamber components. The strike step ensures that the plasma conditions in the chamber reaches some pre-defined acceptable level before actual etching (which typically employs a higher RF power) begins on the substrate in accordance with a predefined etch recipe. Thus, while the strike step may involve plasma conditions that are not suitable for the actual etching, the strike step is nevertheless a very important step in ensuring satisfactory etch results and high device yield per substrate.
In the prior art, the strike step is often executed for some arbitrary period of time in accordance with some pre-defined best known method, or BKM. The strike step duration is typically empirically determined beforehand based on feedback data obtained from test substrates and is executed before the execution of each etch recipe. For example, some BKMs may call for a five second strike step to ensure reliable ignition and stabilization of the plasma prior to etching. The entire five second strike step is typically performed irrespective whether the plasma has been ignited and stabilized in the first, second, third or fourth second of the five-second duration.
If the plasma is ignited and stabilized very early on in the predefined strike step duration, the remaining portion of the strike step duration represents, in essence, wasted time since the plasma has already been ignited and stabilized, and no useful etching occurs during that time. The wasted time reduces the overall throughput of the plasma processing system, leading to a higher cost of ownership for the plasma tool (as a function of units of device produced). Furthermore, the presence of the strike plasma in the chamber during the wasted time contributes to the premature degradation of the chamber components (thereby necessitating more frequent cleaning and maintenance cycles) and/or contributes to the unwanted etching of the substrate without a corresponding benefit in terms of improved and/or increased substrate production.
On the other hand, if the plasma fails to ignite or stay sustained after the expiration of the strike step, the initiation of the main etch step in the absence of a well-characterized plasma often results in damage to the substrate.
In view of the foregoing, there are desired improved techniques for detecting whether the strike step is successful and/or for minimizing the time duration required to perform the strike step.